Reaction processes in the interior of an MgO-carbon brick with boron carbide additive

This paper deals with oxidation protection afforded to carbon-containing refractory materials by boron carbide. Following the previous tests on the stability of B₄C at high temperatures and on the microstructural characteristics of a fired MgO-C brick, now the reactions, which are to be expected in the interior of the brick, in a neutral atmosphere are discussed. The results of the theoretical consideration and the experiments carried out provide evidence that MgO is unstable in the presence of boron carbide as from approx. 1300°C. Similar instability is shown by magnesium borate, which is reduced by carbon as from about 1300°C. In both reactions, reactive gases occur (Mg vapour and B₂O₂), which prevent its ingress and hence oxidation of the carbon by binding the oxygen. As well as the dense layer of borate melt on the brick surface, this phenomenon is obviously responsible for the high efficiency of the boron carbide as an antioxidant for MgO-C materials.     

Study of the oxidation protection of MgO-C refractories by means of boron carbide

Boron carbide is used as a highly effective antioxidant in carbon-containing refractories. The oxidation-inhibiting action of this additive has not been clearly defined. The work first deals in theory with the stability of boron carbide at high temperatures. The practical section presents the reaction sequence as determined by means of thermogravimetric analysis and X-ray diffraction analysis. The tests showed that B₄C oxidizes during firing at below 1000°C, to give magnesium borate (3 MgO · B₂O₃), which is stable at high temperatures.     

添加氧化铝烧结助剂的碳化硼陶瓷无压烧结工艺研究

采用无压烧结工艺,通过添加质量分数为5%的氧化铝烧结助剂,制备得到了碳化硼陶瓷,其中烧结温度从2000℃到2250℃,保温时间为1、2和3 h。对烧结试样进行了体积密度、显气孔率、维氏硬度、显微形貌和晶体结构测试,并与2250℃下烧结得到的不添加烧结助剂碳化硼试样进行了比较。实验结果表明:由于烧结助剂与碳化硼在扩散运动中的相互作用,导致添加氧化铝助剂无压烧结碳化硼晶粒的形态变化具有温度选择性;氧化铝助剂所体现的液相扩散作用和钉扎作用,既可阻碍碳化硼晶粒长大,又可大幅度降低碳化硼的气孔率;通过烧结工艺控制氧化铝助剂成分在晶粒烧结体中的比例,可以将氧化铝成分完全包裹在碳化硼晶粒内部,有利于碳化硼烧结中的晶粒控制和空隙调整,从而避免助剂成分对烧结碳化硼可能造成的不利影响。     

Friction Stir Processing for Enhancement of Wear Resistance of ZM21 Magnesium Alloy

Present work pertains to surface modification of the magnesium alloy using friction stir processing (FSP). Silicon carbide and boron carbide powders are used in the friction stir processing of the ZM21 Magnesium alloy. Coating was formed by FSP of the alloy by placing the carbide powders into the holes made on the surface. Surface coating was characterized by metallography, hardness and pin-on-disc testing. Friction stir processed coating exhibited excellent wear resistance and is attributed to grain boundary pinning and dispersion hardening caused by carbide particles. Surface composite coating with boron carbide was found to possess better wear resistance than coating made with silicon carbide. This may be attributed to formation of very hard layer coating of boron carbide reinforced composite on the surface of magnesium alloy. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on ZM21 Mg alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly shows that wear resistance of friction stir processed composite layer is better than that of mild steel and stainless steel. This work demonstrates that friction stir processing is an effective strategy for enhancement of wear resistance of magnesium alloys.     

Friction Stir Surfacing Route: Effective Strategy for the Enhancement of Wear Resistance of Titanium Alloy

Titanium alloys are widely used in aerospace applications due to their properties like high strength to weight ratio, good corrosion and creep resistance. Poor wear resistance of these alloys limits their use in tribological applications. Friction surfacing technique is now recognized as an effective solution to surface engineer the light weight high strength alloys to make them suitable for general engineering applications involving wear and corrosion. The present work pertains to a study on wear resistance of surface coating of boron carbide on Ti–6Al–4V alloy using friction surfacing technique. Coating was formed by placing the boron carbide powder into the holes of predetermined depth on the surface and was characterized by metallography, electron probe micro analysis and dry sliding wear testing. The present study revealed that titanium alloy could be friction surfaced with boron carbide powder. The coating exhibited excellent wear resistance, which is attributed to the formation of strong metallurgical bond with the substrate. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on titanium alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly revealed that wear resistance of friction stir surfaced composite layer is better than that of mild steel and stainless steel. This study demonstrated that friction stir surfacing is an effective strategy for the enhancement of wear resistance of titanium alloys.     

Structure formation during activated sintering of high-speed steel

Features of structure formation during activated sintering of high-speed steel using a boron-containing additive as an activator are studied. It is shown that presence of an activating additive, its composition and content in the charge, determine features of structure formation compared with sintering without an additive. In the temperature range for solid-phase sintering the content of additive in the charge and the ratio of its components have little effect on the average carbide component size. As temperature increases and there is an increase in boron content in the charge sintering is accompanied by both some increase in austenite grain size and development of a significant difference in carbide grain size. The effect of nickel on size reduction of sintered steel structural components is established.     

渗硼剂粒径对钛合金渗硼层组织和性能的影响

采用不同粒径的碳化硼作为硼源,以氧化铈作为催渗剂,在非惰性气氛保护下对TC4钛合金表面进行固体渗硼。利用场发射扫描电镜(FESEM)、背散射电子成像(EBSD)、X射线衍射(XRD)、UMT摩擦磨损试验机对渗硼层的微观组织结构和摩擦学性能进行研究。结果表明,TC4钛合金表面渗硼层为双相硼化物;随着碳化硼粒径的增加,Ti B层厚度与渗层总厚度减少,渗层硬度约为60(HR30N),硼化物层摩擦系数约为0.2,碳化硼的杂质组成对渗层有较大影响。     

提钒转炉用MgO--Fe--C砖性能研究

为延长MgO-C砖在提钒转炉上的使用寿命,本研究开发了一种新型MgO-Fe-C砖,通过与传统的MgO-C砖进行对比研究,考察这种新型耐火材料的使用性能.研究结果表明:在1400℃的使用温度下,导致提钒转炉用MgO-C砖使用寿命短的原因是脱碳层的烧结性差,抗冲刷性不理想;而对于本研究所开发的MgO-Fe-C砖,铁粉在氧化层氧化及使用条件下原位形成MgO-FeOss,有效地改善脱碳层的烧结性能,并形成致密且高结合强度的脱碳层,显著地提高了耐火材料的抗熔渣侵蚀性和抗氧化性,有利于耐火材料寿命的提高,因此MgO-Fe-C砖是具有良好应用前景的提钒转炉用MgO-C砖的替代品.      

The effect of Fe addition on the densification of B4C powder by spark plasma sintering

Boron carbide is a low-density ceramic with high hardness and stiffness values that make it a valuable candidate for light armor applications. Fully dense boron carbide is fabricated by hot pressing of fine (<2 µm) powder at a relatively high temperature (2150–2200°C). Fully dense boron carbide can be processed from an initial mixture of 5.5 vol.% Fe and low-cost B₄C powder by spark plasma sintering (SPS) at 2000°C. At this temperature, Fe-free boron carbide can be consolidated only to 96% of the theoretical density. The effect of the Fe addition on the densities is even more pronounced at lower processing temperatures and is related to the presence of a liquid phase in the Fe-containing material. The resulting microstructure and mechanical properties of the Fe-containing boron carbide are presented and discussed.     

低碳镁碳砖抗氧化剂的研究

研究了Al、Si和B4C几种抗氧化添加剂对低碳镁碳砖抗氧化性能的影响.利用热重法分析了加热过程中耐火材料试样的失重率;测定耐火材料的脱碳层厚度;探讨了抗氧化添加剂的作用机理;并明确了适合于低碳镁碳砖用抗氧化添加剂的种类.研究结果表明,与添加Al和Si相比,添加B4C能更好地提高低碳镁碳砖的抗氧化性能.通过在低碳镁碳砖中...     

DISPERSIONS OF BORON CARBIDE WITH BARRIER LAYERS IN STAINLESS STEEL

Abstract

The possibility has been studied of preventing reaction between boron-carbide particles and an austenitic stainless-steel matrix by means of a a barrier layer on the particles. Silicon carbide is compatible with boron carbide up to 2000°C but reacts extensively with austenitic stainless steel above 1000°C and is thus ineffective as a barrier layer. Titanium carbide deposited from the vapour phase, although it reacts with B₄C above 1300°C and under certain conditions with austenitic stainless steel at 1100°C, was the most suitable material considered.     

Stabilization of lactate dehydrogenase following freeze thawing and vacuum-drying in the presence of trehalose and borate

PURPOSE: The purpose of this work was to investigate the effects of trehalose and trehalose/sodium tetraborate mixtures on recovery of lactate dehydrogenase (LDH) activity following freeze-thawing and centrifugal vacuum-drying/rehydration. The storage stability of LDH under conditions of either high relative humidity or high temperature was also studied. METHODS: LDH was prepared in buffered aqueous formulations containing trehalose alone and trehalose/"borate" mixtures. Enzymatic activity was measured immediately following freeze-thawing and vacuum-drying/rehydration processes, and also after vacuum-dried formulations were stored in either high humidity or high temperature environments. Also, glass transition temperatures (Tg) were measured for both freeze-dried and vacuum-dried formulations. RESULTS: The Tg values of freeze-dried trehalose/borate mixtures are considerably higher than that of trehalose alone. Freezing and vacuum-drying LDH in the presence of 300 mM trehalose resulted in the recovery of 80% and 65% of the original activity, respectively. For vacuum-dried mixtures, boron concentrations below 1.2 mole boron/ mole trehalose had no effect on recovered LDH. After several weeks storage in either humid (100% relative humidity) or warm (45 degrees C) environments, vacuum-dried formulations that included trehalose and borate showed greater enzymatic activities than those prepared with trehalose alone. We attribute this stability to the formation of a chemical complex between trehalose and borate. CONCLUSIONS: The high Tg values of trehalose/borate mixtures offer several advantages over the use of trehalose alone. Most notable is the storage stability under conditions of high temperature and high relative humidity. In these cases, formulations that contain trehalose and borate are superior to those containing trehalose alone. These results have practical implications for long-term storage of biological materials.     

Tests on the oxidation-inhibiting effect of CaB6 in refractory MgO-C materials

The oxidation-inhibiting action of B₄C as an antioxidant is to be attributed to two possible mechanisms. The first describes the formation of reactive gases (Mg vapour and B₂O₂) that react with oxygen reducing its partial pressure in pores of the material. The second mechanism results in the formation of a borate layer which includes carbon between the oxide grains and thus protects it from further oxidation. The present study made an examination as to whether these mechanisms also play a part in the use of CaB₆. The results confirm the pattern of chemical behaviour in MgO-C materials with boron-containing antioxidants recorded in previous tests. However, the temperature at which the protective effect begins lies in this case at about 200°C lower when compared to B₄C. It is obviously due to the formation of a eutectic melt in system MgO-CaO-B₂O₃ at approx. 1080°C.     

A study on the mechanism of reaction between refractory materials and aluminium deoxidised molten steel

The conditions of formation of inclusions in steel during ladle refining in MgO-C lined ladles have been investigated in laboratory experiments at 1873 K in MgO-C crucibles with 50 g iron. From both the experimental results as well those of a thermodynamic study conducted in parallel, the following conclusions can be made about the reaction mechanism between the MgO-C refractory and aluminium deoxidised molten steel: At steelmaking temperature an internal oxidation-reduction occurs in the MgO-C refractory. The formation of a thin oxide layer at the interface is due to the reaction between magnesium vapour and aluminium dissolved into the molten steel and the CO(g) generated by the reaction between MgO and C in the crucible walls. The oxide inclusions formed in the steel have been shown mainly to consist of MgO, Al₂O₃ or mixture of them. Some of the finest inclusions are considered to be formed as secondary inclusions during cooling and solidification of the steel and they are connected only with the diffusion of magnesium from the crucible to the molten steel. The thermodynamic calculations indicate that during vacuum refining, as the pressure decreases, the chemical compatibility of carbon-bonded magnesia for ladle lining decreases.     

Sintering of boron carbide containing small amounts of free carbon

Conclusions A study was made of the sintering kinetics of grade ch. boron carbide, technical boron carbide cleaned of impurities, and boron carbide synthesized from the elements. An investigation was carried out also into the reactive sintering of a mixture of boron and carbon black. Reactive sintering fails to yield dense parts in boron carbide. The best sinterability is exhibited by a fine technical boron carbide powder cleaned of free carbon and other contaminants. Parts with a porosity of less than 5% can be produced from such a powder by pressing and subsequent sintering. However, it is not the presence of free carbon that controls the sintering behavior of boron carbide, since pure and synthesized boron carbide powders containing no free carbon are characterized by poor sinterability. The high activity of the technical powder is probably linked with the presence of structural defects and stresses generated in the course of its manufacture, during milling and quenching. The annealing spectrum of these defects covers a wide temperature range, and consequently the energy of activation for the densification of boron carbide steadily grows with increase in shrinkage.Translated from Poroshkovaya Metallurgiya, No. 7 (151), pp. 27–31, July, 1975.     

多孔碳化硼的力学性能及其微观组织

采用无压烧结法制备得到了多孔的碳化硼,用扫描电镜研究了该材料的微观组织;并测定了不同孔隙率碳化硼的抗弯强度和密度,分析了多孔碳化硼的孔隙率与抗弯强度和密度的关系。研究结果表明:采用无压真空烧结法所制备的多孔碳化硼材料的微观组织烧结良好,烧结颈明显,孔隙大小比较均匀;当空隙率为30%时,所制备的多孔碳化硼密度为1.714 g/cm3,抗弯强度为100.85 MPa。     

转炉渣与镁质耐火材料的润湿机制

 采用静滴法研究了2种情况下转炉渣与炉衬耐火材料的润湿性。当转炉渣与耐火材料直接接触时,转炉渣不熔化,当用刚玉坩埚将两者隔开时,界面处发生反应性润湿,动态接触角随温度升高而单调减小,铺展面积随时间成线性增加。渣中FeO含量由于镁碳砖基板表面的碳及中间产物CO的还原而降低,导致炉渣熔化性温度升高而未熔化。液态熔渣沿镁碳砖表面的气孔和裂纹向基体内扩散、渗透,渣中铁氧化物与镁碳砖机体内的MgO反应生成含高熔点相的黄色渗透层,起到保护炉衬的作用,从而解释了炉渣与耐火材料的粘附机制,为优化溅渣护炉工艺,合理调整炉渣成分和选择耐火材料等工艺操作提供理论依据。     

硼对耐磨球墨铸铁组织和力学性能的影响

研究了不同硼含量对含碳3．4％－3．8％、含硅2．0％－3．0％，含锰2．0％－＝3．0％的球墨铸铁铸主热处理态组织和力学性能的影响。结果表明：随着硼含量的增加，铸态组织中的碳化物数量增加，硬度提高，冲击韧性下降，当硼含量大于0．010％时，铸态组织中出现了明显的网状或断网状碳化物，冲击韧性下降幅度较大，经淬火、回火后、碳化物发生不同程度的溶解。当硼含量为0．003％－0．0105时，热处理后碳化物基本消失，硬度保持在HRC50左右，冲击韧性达到或超过9J／cm^2；当硼含量超过0．010％时，热处理后仍存在断网状或条块状碳化物，硬度稍有增加，冲击初性明显下降。     

Influence of the synthesis conditions of boron carbide on its structural parameters

Boron carbide is prepared by self-propagating high-temperature synthesis (SHS) in a composition range from 5 to 30 at % carbon. The introduction of an inert (MgO) and active (Mg(ClO₄)₂) additives into the reaction mixture leads to a variation in process parameters such as the temperature and combustion rate. It is established that the unit cell metrics of boron carbide substantially vary depending on the synthesis conditions. The degree of the effect of the SHS mode on the crystal structure rises with an increase in the carbon fraction in the boron carbide structure. This regularity is associated with the ordering diversity of carbon atoms in nonstoichiometric boron carbide. No influence of the synthesis conditions on the unit cell parameters is observed for stoichiometric boron carbide, which is associated with the structure saturation with carbon. It is shown that the variation in the combustion temperature during SHS of boron carbide of the same composition leads to the variability of the structural parameters, thus reflecting the influence of the synthesis conditions on the material crystal structure.     

不同碳含量的MgO-C耐火材料与超低碳钢液的相互作用

研究了3种C含量的(3%、5%、10%,质量分数)MgO-C耐火材料与超低碳钢的相互作用。利用ICPAES、氧氮分析仪、碳硫分析仪检测了与实验MgO-C耐火材料接触的钢液的成分,用XRD分析了耐火材料反应前后的物相变化,并利用SEM观察了耐火材料/钢界面。结果表明,随着镁碳耐火材料中C含量的增加,耐火材料/钢界面附近的渗透层厚度增加;反应后钢液中的C、N、Al含量以及Mg含量随着耐火材料中C含量的增加而增加,钢中O含量随之降低;反应前后的镁碳耐火材料都有镁铝尖晶石的存在,高C含量的耐火材料反应后镁铝尖晶石含量增加,因此低碳镁碳耐火材料更有利于超低碳钢的生产。